SECTION : Faraday's Law

SUBJECT:
The Falling Magnet with Finite Resistance Ring

DESCRIPTION:

Here we show an example of Faraday's Law, as a magnet
is dropped through a conducting ring. As the magnet
falls under gravity, a current is induced in the ring
that tries to keep the magnetic flux through the area
of the ring constant. This corresponds to a field that
produces a force that opposes the motion of the magnet:
as it approaches from above, the induced current generates
a field that pushes the magnet upwards. Once
it falls through the ring, the direction of the current
changes to produce a field that tries to pull
the magnet upwards. Since the resistance of the ring
is finite (but small) and the magnet is heavy, the flux
through the ring does not stay constant, and the relatively
small magnitude of the induced field does not generate
a force strong enough to prevent the magnet from falling
through.

The first animation shows the magnetic field configuration
around a magnet as it falls. The current in the ring
is indicated by the small moving spheres. The motions
of the field lines are in the direction of the local
Poynting flux vector.

The video (second and third image) shows this experiment
actually being conducted in real life. In order to make
this phenomena easily observable, the copper ring has
been cooled in liquid nitrogen to bring down its resistivity.

The third animation shows a three-dimensional fieldline
representation of the same thing.